1. Field of the Invention
The present invention relates to a clutch slip control device, a method of slip control, a method of manufacturing the slip control device, and a vehicle control system applicable thereto. More specifically, the invention pertains to a system of outputting a plant input of clutch operation to make an actual slip revolution speed coincident with a target slip revolution speed and adjusting slip conditions based on the plant input thus output.
2. Description of the Related Art
A variety of known clutch slip control devices include those for controlling slip conditions of a lock-up clutch of a torque converter. Such slip control devices are designed to solve contradictory problems, that is, connection of inputs with outputs of the torque converter transmits the vibrations of an engine directly to a transmission in a range of low engine speed, thus worsening the riding comfortableness whereas disconnection of inputs and outputs in a wide range of engine speed prevents sufficient reduction of the fuel consumption rate.
Improvement has been conventionally given to these slip control devices to reconcile the high response and control stability. One example of proposed improvement includes a process of calculating a current plant input based on a deviation of the actual slip revolution speed from a target slip revolution speed and a differential and an integral of the deviation or a differential and a second differential of the deviation (JAPANESE PATENT PUBLICATION GAZETTE No. H2-586). Another example includes a process of expanding these quantities in time series to calculate an increase in the plant input (JAPANESE PATENT LAYING-OPEN GAZETTE No. S64-30966). By matching the characteristics of the control devices to those of a plant or slip adjusting mechanism, these improved control devices can stably regulate and maintain the slip quantity equal to or close to a target value and realize the high follow-up ability over the target value without lowering the stability.
These control devices, however, still have a drawback, that is, insufficient control over characteristic perturbations in a control system of clutch slip conditions. When there is a significant difference in properties between individual lock-up clutches and slip-regulating hydraulic control systems or when deterioration of frictional material or operating oil varies the frictional characteristics of the lock-up clutch or .mu.-v characteristics of the clutch from the initial design conditions to damage the stability of the slip revolution speed, the conventional control devices can not stably or rapidly control the slip quantity equal to or close to the target value. This problem is described more in detail with the drawings of FIGS. 37 through 39.
Characteristic perturbations of the slip control system are illustrated as the gain and phase of a transfer function from a plant input to a slip revolution speed. FIG. 37 is a graph showing a difference in design properties between individual clutches. The characteristics of clutches are differed by instability or pressing orientation of the frictional material especially in a high frequency domain. When the frictional material is worn or operating oil deteriorates thermally over time, both the gain and phase characteristics lower from the initial design properties in medium and high frequency domains as shown in FIGS. 38A and 38B. The frictional characteristics may have a resonance peak in the high frequency domain as illustrated in FIG. 39. Under such conditions, the frictional material may display the so-called stick-slip behavior (repetition of contact, revolution, and separation) and further cause self-oscillation. This results in total damage of the control stability of the slip revolution speed.
The above problem is ascribed to the design principle of the conventional control devices. These control devices are designed to satisfy the stability and follow-up properties of control under only specific control conditions or on the assumption that the control characteristics of the clutch are not significantly varied. Possible improvement given to the control device which calculates the current plant input based on the deviation of the actual slip revolution speed from a target slip revolution speed and the differential and second differential of the deviation is changing the constants of control expressions according to the characteristic perturbations of the control system. Such improvement, however, makes the structure of the control system undesirably complicated while not ensuring the stability of the change-over algorithm.
The problems of characteristic perturbations always exists in real systems. In order to guarantee the stability of control, the actual control devices have only the slow response. Although the slow response control stably maintains the slip revolution speed substantially equal to the target slip revolution speed under stationary driving conditions, it can not block the input torque variations of the engine or enhance the transmission efficiency of the torque following the transient driving conditions which causes a variation in the target slip revolution speed.
Another problem addressed by the invention is the stability of a real control system. Although a variety of control principles based on the control theories are available presently, digital control with a microcomputer is generally applied to a slip control system. The microcomputer-based control in a finite time scale causes several problems, quantizing errors in data processing, errors in processing of operation data, and noise errors included in input data. It is accordingly important to guarantee the stability over such errors. The fixed decimal point operation which is often applied to shorten the operation time drastically lowers the precision of small data obtained by the operation while causing an overflow of large numerical data to make the operation results totally useless. In many cases, the floating decimal point operation can not be applied because of the limited microcomputer functions and the restricted operation time. Prevention of divergence of oscillation of control and guarantee of the stability are essential factors in design of the actual control device.